The present invention relates to a nitride semiconductor laser device having a low lasing threshold current and excellent noise characteristics, and relates to an optical device using the nitride semiconductor laser device.
A nitride semiconductor laser device having a light emitting layer of InGaN crystal is reported in IEICE TRANS. ELECTRON., Vol. E83-C, No. 4, April 2000, pp. 529-535.
Japanese Patent Laying-Open No. 10-270804 discloses a nitride semiconductor device having a light emitting layer formed of GaNAs, GaNP or GaNSb crystal.
Japanese Patent Laying-Open No. 10-261838 discloses a gallium nitride based semiconductor light-emitting device having a quantum well active layer sandwiched between nitride semiconductor cladding layers and/or guiding layers. The quantum well active layer, which has a layered structure consisting of well and barrier layers, is formed of nitride semiconductor containing at least indium and gallium, specifically InGaN. The number of well layers is 2 to 4 and the thickness of the barrier layer is 4 nm or less. It is also disclosed that the light-emitting device with the structure as described above has a ridge structure with a stripe width of 1 to 5 xcexcm.
Japanese Patent Laying-Open No. 11-214788 discloses a gallium nitride based semiconductor laser device having an active layer sandwiched between nitride semiconductor cladding layers and/or guiding layers. The active layer is formed of nitride semiconductor, current is injected into a stripe region with its width smaller than that of the active layer. The width of the stripe is 0.2 to 1.8 xcexcm. Regarding the material of the active layer, the following description is found in this document. xe2x80x9cThe active layer having a multiple quantum well structure consists of two quantum well layers of In0.2Ga0.8N and one barrier layer of In0.05Ga0.95N. The respective compositions are set depending on the required laser emission wavelength. The In ratio of the quantum well layer may be increased for attaining a longer emission wavelength while that may be decreased for attaining a shorter emission wavelength. The quantum well and barrier layers may be formed of mixed crystal semiconductor of four or more element mixed crystal that is based on InGaN to which a slight amount of another element is added. The barrier layer may be of GaN only.xe2x80x9d However, this document shows nothing about specific elements except for In, Ga and N to be used for the active layer. Moreover, the width of the stripe region defined by this prior art is only applied to the active layer of InGaN.
In the conventional nitride semiconductor laser device with the light emitting layer of InGaN crystal, the electrons and holes in the semiconductor material of the light emitting layer have considerably great effective mass. Therefore, the lasing threshold current can only be reduced to a limited extent.
On the other hand, the nitride semiconductor laser disclosed in Japanese Patent Laying-Open No. 10-270804 includes the light emitting layer of GaNAs, GaNP or GaNSb crystal, and accordingly the effective mass of the electrons and holes may be made smaller than that of the conventional InGaN crystal, which suggests that population inversion for producing laser emission can be achieved with a lower carrier density (lasing threshold current can be reduced). However, this document discloses only a semiconductor laser device having a mesa structure of 5 xcexcm or 20 xcexcm in width as a specific example. In reality, the shape and dimension of the semiconductor laser structure have great influences on laser performance such as the lasing threshold current. This document, however, shows nothing about the relation between such a shape and dimension and laser performance.
An object of the present invention is to provide a nitride semiconductor laser device having a low threshold current and low noise by providing a specific structure to enhance the performance of the semiconductor having a light emitting layer formed of As, P or Sb containing nitride semiconductor.
The inventors of the present invention have found that in a semiconductor laser including a light emitting layer formed of nitride semiconductor containing at least one of As, P and Sb, the maximum width through which current is injected to the light emitting layer via a p-type layer considerably contributes to reduction of the threshold current. Moreover, the inventors have found that the maximum width ranging from 1.0 xcexcm to 4.0 xcexcm makes it possible to lower the threshold current and the maximum width ranging from 1.0 xcexcm to 3.5 xcexcm provides a self oscillation characteristic, as detailed later.
Here, xe2x80x9cthe maximum width through which current is injected to the light emitting layer via a p-type layerxe2x80x9d refers to the maximum width, through which current is injected, viewed from the end surface of the resonator of the semiconductor laser. For example, a semiconductor laser with a ridge stripe structure as shown in FIG. 7(a) has xe2x80x9cthe maximum width through which current is injected to a light emitting layer via a p-type layerxe2x80x9d corresponding to the width (W) of the ridge stripe portion. For a ridge stripe having a trapezoidal cross section as shown in FIG. 1, xe2x80x9cthe maximum width through which current is injected to a light emitting layer via a p-type layerxe2x80x9d corresponds to the width of the bottom (maximum width) of the trapezoid. For a semiconductor laser having a current blocking layer for limiting the width through which current is injected to the light emitting layer as shown in FIG. 7(b), xe2x80x9cthe maximum width through which current is injected to the light emitting layer via a p-type layerxe2x80x9d corresponds to the maximum width (W) between the opposing portions of the current blocking layer.
Additionally, the inventors have found that the distance from the boundary between the light emitting layer and the p-type layer to the bottom of the ridge stripe or current blocking layer also contributes to reduction of the threshold current. It is further found that such a distance of 0-0.3 xcexcm or 0.01-0.3 xcexcm can provide a lower threshold current.
Referring to FIG. 7(a), in the semiconductor laser having the ridge structure, the distance from the boundary between the light emitting layer and the p-type layer to the bottom of the ridge stripe corresponds to xe2x80x9cthickness (d) of the residual film (residual film thickness).xe2x80x9d Referring to FIG. 7(b), in the semiconductor laser having the current blocking layer, the thickness (d) from the boundary between the light emitting layer and the p-type layer to the current blocking layer contributes to reduction of the threshold current, and this thickness (d) is also hereinafter referred to as xe2x80x9cresidual film thickness.xe2x80x9d
Accordingly, the present invention is directed to a nitride semiconductor laser device including n-type and p-type layers made of nitride semiconductor and formed on a substrate and a light emitting layer arranged between the n-type and p-type layers. The light emitting layer is formed of a well layer or a combination of well and barrier layers. At least the well layer among the constituent layers of the light emitting layer is made of nitride semiconductor containing element X, N and Ga, wherein element X is at least one selected from the group consisting of As, P and Sb. In the nitride semiconductor containing the element X, N and Ga, X has an atomic fraction smaller than that of N. A maximum width through which current is injected into the light emitting layer via the p-type layer is from 1.0 xcexcm to 4.0 xcexcm.
The present invention is also directed to a nitride semiconductor laser device including n-type and p-type layers made of nitride semiconductor and formed on a substrate and a light emitting layer arranged between the n-type and p-type layers. The light emitting layer is formed of a well layer or a combination of well and barrier layers. At least the well layer among the constituent layers of the light emitting layer is made of nitride semiconductor containing element X, N and Ga, wherein element X is at least one selected from the group consisting of As, P and Sb. In the nitride semiconductor containing the element X, N and Ga, X has an atomic fraction smaller than that of N. A maximum width through which current is injected into the light emitting layer via the p-type layer through is from 1.0 xcexcm to 3.5 xcexcm. The laser device has a self oscillation characteristic.
In the present invention, the light emitting layer may have a single quantum well structure formed of one well layer only or barrier layer/well layer/barrier layer, or the light emitting layer may be composed of a plurality of well layers and a plurality of barrier layers.
Preferably, when the nitride semiconductor laser device has a ridge structure, the distance from the boundary between the light emitting layer and the p-type layer to the bottom of the ridge stripe is from 0 xcexcm to 0.3 xcexcm.
Preferably, when the nitride semiconductor laser device has a current blocking layer for limiting the width through which current is injected into the light emitting layer, the distance from the boundary between the light emitting layer and the p-type layer to the current blocking layer is from 0.01 xcexcm to 0.3 xcexcm.
In a preferred manner of the present invention, the substrate of the laser device is made of nitride semiconductor crystal or has a structure comprising a nitride semiconductor crystal film with a dislocation density of at most 107/cm2 grown on another crystal material.
According to the present invention, an optical device using the nitride semiconductor laser device as defined above is provided.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.